1. Field of the Invention
The present invention relates to a method of producing a frame body for an electric motor provided with heat radiation fins.
2. Background Art
The operation of electromagnetic apparatuses causes a temperature rise due to internal power loss. In an electric motor, a plurality of radiation fins are attached onto the outer circumferential surface of the frame body to increase the surface area of the frame body in contact with air. The heat radiation fins function to release heat produced in the motor to the outside to thereby prevent the temperature rise.
The motor frame with the heat radiation fins has been prepared in accordance with manufacturing steps as shown in FIG. 5. Namely, a rectangular or square flat plate 1 having a predetermined size (shown in FIG. 3a) is bent into a generally round shape by subjecting it in a rolling machine having two or three rolls (not shown), as shown in FIG. 3b. The generally round-shaped body is forwarded to a welding section where the opposing edge surface portions of the round-shaped body are brought to contact with each other and they are bonded by welding in the longitudinal direction to thereby form a cylindrical frame body 2 (FIG. 5c). The cylindrical frame body 2 is then subjected to a plastic process for regulating the shape. Namely, a plurality of expansion pawl members 3 is inserted in the frame body 2 and are extended in the radial direction, whereby the inner circumferential surface of the frame body 2 is expanded in the radial direction so as to render the inner diameter of the frame body to be a nearly true circle and to make an inner diameter suitable for forcibly inserting a stator core which is described below (FIG. 5d). Then, as shown in FIG. 5e, heat radiation fins 11 formed in a U-shaped in cross section are attached onto the outer circumferential surface of the frame body 2 by projection welding. Thus, the frame body 2 with a plurality of heat radiation fins 11 is prepared.
A stator core 6 with a number of slots 5 in which coils (not shown) are inserted is forcibly fitted into the frame body 2 (FIG. 5f) to thereby fabricate a stator as shown in FIG. 6.
During the operation of the motor having the stator 6, heat produced from the stator core 6 is transmitted from the contacting area 7 between the stator core 6 and the frame body 2 through the frame body 2 and the welded portions 8 of the heat radiation fins 11 to the heat radiation fins 11; thus the heat is released outside.
In the electric motor of this type, it is necessary to increase the surface area of the contacting part 7 and the surface area of the welded portions 8 in order to increase an amount of heat radiation produced in the stator core 6.
However, when the surface area of the welded portions 8 is increased, much heat resulting from the welding is absorbed in the frame body 2. Accordingly, the shape of the inner diameter portion of the frame body 2 is deformed into a substantially oval shape due to thermal stress caused by the welding. In addition, the frame body 2 itself expands in the circumferential direction at positions near the welded portions 8.
Accordingly, in the motor frame prepared by the conventional method, the surface area between the frame body 2 and the stator core 6 is small with the consequence that thermal resistance of contact increases depending on the small surface area of contact, whereby thermal transmission properties from the stator core 6 to the heat radiation fins 11 is reduced. As a result, the function of the heat radiation fins 11 to prevent temperature rise of the coils of the electric motor is reduced.